Crystal chemistry of povondraite by SCXRD, EMPA, Mössbauer spectroscopy and FTIR

Five povondraite crystals from San Francisco Mine, Villa Tunari, Bolivia, have been structurally and chemically characterized by single-crystal X-ray diffraction and electron microprobe analysis. For the first time, this characterization is accompanied by Mössbauer spectroscopic and single-crystal infrared spectroscopic data, which show the exclusive presence of Fe3+ at both the octahedrally-coordinated Y and Z sites as well as slight disorder of (OH) and O over the O(1) and O(3) sites. The data obtained along with those for earlier-studied bosiite and oxy-dravite oxy-Tourmalines show a complete substitution series described by the reaction YFe3+3 + ZMg + ZFe3+4 ↔ YAl2 + YMg + ZAl5 (i.e. Fe3+Al-1) with variation of the structural parameters dominated by Fe3+ (or Al). Povondraite is the tourmaline member having the largest unit-cell parameters due to the larger size of Fe3+ relative to other trivalent cations (V > Cr > Al). In the tourmaline-supergroup minerals, the a and c unit-cell parameters vary from ∼15.60 A to ∼16.25 A and ∼7.00 A to ∼7.50 A, respectively. Their values increase with increasing Fe3+ or decreasing Al. Endmember compositions related to the smallest and largest a-and c-parameters are, respectively, NaAl3Al6(Si3B3O18)(BO3)3(OH)3(OH) (synthetic tourmaline) and NaFe3+3(Fe3+4Mg2)(Si6O18)(BO3)3(OH)3O (povondraite).